Time for a new board game design attempt. This time I'm going to try
documenting my design process, mostly to motivate myself to actually
finish the design this time. The inspiration was the recent spate of
lighter variations of the 18xx system,
none of which really does what I want.
If you're not familiar with 18xx, this post probably won't make much
sense, and you might as well stop reading. Sorry.

My goals:

Strip away everything possible from an 18xx game except for the train
rush, but try to retain the feel of the rush. Lokomotive Werks
tried to do something along these lines, but I didn't really care for
it for various reasons.

Aim for a 90-minute game for the first play.

Fully deterministic with no hidden information.

It's a plus if the mechanisms make more sense for something that isn't
train-themed. (Some people I play with have an allergy to trains).

First decision that had to be made was whether players represent
companies or investors that might control several companies. The
latter seemed much more likely to create interesting turn order and
timing issues. Features like collusion between the same player's
companies are pretty central to the way a 18xx train rush unfolds.
And the possibility of controlling multiple companies also creates the
delicious dilemma of companies being both assets and liabilities.

But having companies be separate entities has the problem that the
game then also needs to keep at least a limited form of share dealing
in the game. This is in conflict with both of the first goals.

After a couple of days of doodling around with the game I hit upon the
idea of keeping the companies, but totally gutting the 18xx stock
round. The only thing that you can do on a stock action would be to
start a company: set the par price, buy 50-100% of the company, which
will then receive 10*par price as initial capital. No selling or
buying of shares. The latter would allow cutting out the stock market
entirely.

For operating companies I absolutely did not want any kind of board
play. Again the goal is to cut ruthlessly on the first pass. The
simplest possible solution would be to have each train produce a
constant income every turn until it rusts (e.g.
Railroad Barons),
but that felt a little too spartan for me. There should be something
players can do to differentiate two identical companies.

My first stab at this is that train won't necessarily produce
income every turn. Instead when you first buy one, you put 1-3 tokens
on top of it. When the company operates, flip one token on each train.
Any train that has all the tokens on it flipped produces income that
grows non-linearly in the number of tokens. For one token $x,
for two tokens $2.5x, for three tokens $4.5x. After a train has
produced income, populate it with new tokens (can be different amount
from original).

This seemed like a system with some potential: mechanically still
very simple, but offering some interesting decision space when
it came to the number of tokens you place on a train. You'd much
rather prime a train with 3 tokens once than with 1 token thrice.
But maybe you need the money sooner (or at least an option for
having the money). And how long will the train live anyway? Somebody
being too optimistic with their scheduling might even prompt other
players to short-schedule their trains, withhold the money, and
accelerate the train-buying.

As a bonus this also satisfied goal 4. This mechanism makes no thematic
sense with trains, but the whole "small return safely and quickly" vs.
"large risky payoff later" should have some resonance with other
themes. The first couple of possibilities that popped into my mind were
technology companies (where the "trains" would represent R&D projects)
and trade fleets in the Age of Exploration. I'm sure there are more.

Other than that the structure of a single 18xx operation round seems
ok. First run trains, then give a dividend or withhold the income
(though no stock price effect), finally buy trains either from
the bank at face value or from another company controlled by the
same player. Every company must always own a train. If it can't afford
one, then the controlling player will pay for one (this might not
work in practice given shares can't be sold: it might be too easy to
go bankrupt). There should probably also be a train limit.

I figure the game should have a constant alternation between an SR and
sets of 2 ORs, and since it's explicitly a train rush game, using
either the 1844 or 18FR-RCE rule for the foreigners buying a train
from the bank at the end of each set of ORs.

That was enough for a first draft, so I plugged in some numbers. The
train schedule and prices were taken from 1856 and made up an arbitrary
payoff table for the different kinds of trains. Finally I set a par price
range of $40-$100 and started the players with $300 (so the first choice
of the game would be deciding which of 50-70% to go for).

I soloed half a game with these rules, and the game seemed to have the
right kind of feel. Both the decisions on how many trains to buy each
turn and how to run them seemed non-obvious, and there was the same
kind of wheels within wheels planning.

The numbers I'd chosen were of course badly off, but that was to be
expected and can be tuned. There were a number of worrying issues
though:

There probably isn't a runaway winner problem, but there might be a
spiral of death problem. If someone ends up running a company with fewer
trains the opponents, they don't have the usual possibility of
investing into the opponents' companies to boost their income. They
are also unlikely to be able to found a new company very soon, and
are thus much more likely to have dead cash after an SR. Possibly it
should be possible to sell shares (down to 20%?) for the original price
as a stock action.

A totally fixed company operating order is too restrictive and
predictable, there needs to be some way to manipulate it. I think this
should be done in the SR, ideally making stock priority more
important. Perhaps something along these lines: there's a linear turn
order track. A company starts on a space based on its par price
(there's probably a couple of extra spaces between each par price
space). As a stock action a player can move one token up or down on
the track. This costs money: the nth token movement by any player on
a SR costs fib(n).

I also suspect some company differentiation might need to be added later
on. The current idea is that some companies get bonuses for
running trains of a certain level.